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THE  EFFECT  OF  SHORT  ELECTROMAGNETIC 
WAVES  ON  A BEAM  OF  CATHODE  RAYS 


BY 

CLAUDE  JEROME  LAPP 

A.B.  Albion  College,  1917 
A.M.  University  of  Illinois,  1920 


THESIS 

SUBMITTED  IN  PARTIAL  FULFILLMENT  OF  THE  REQUIREMENTS 
FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY  IN  PHYSICS 
IN  THE  GRADUATE  SCHOOL  OF  THE  UNIVERSITY 
OF  ILLINOIS,  1922 


URBANA,  ILLINOIS 


Digitized  by  the  Internet  Archive 

in  2015 


https://archive.org/details/effectofshorteleOOIapp 


UNIVERSITY  OF  ILLINOIS 


THE  GRADUATE  SCHOOL 


May 


192JL 


I HEREBY  RECOMMEND  THAT  THE  THESIS  PREPARED  UNDER  MY 
SUPERVISION  BY CLAUDE  JFDn;.:^  lap? 

ENTITLED,.  THE,„_EFEUCT  OF.  SHORT  ELECTRO.;!  AONET I C WAVES  00  A 

eea::  of  cathode 


BE  ACCEPTED  AS  FULFILLING  THIS  PART  OF  THE  REQUIREMENTS  FOR 
THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY  IT  PHYSICS 


Recommendation  concurred  in* 


Committee 


on 


Final  Examination* 


TABLE  OF  CONTENTS 


I INTRODUCTION  1 

II  DESCRIPTION  OF  APPARATUS  2 

III  OPERATING  CONDITIONS  14 

IV  PHOTOGRAPHIC  MANIPULATION 17 

V MEASUREMENTS IS 

VI  RESULTS 30 

VII  DISCUSSION 41 

VIII  SUMMARY 46 


I INTRODUCTION 


Some  years  ago  J.J.  Thomson1 * 3  advanced  a theory  of  light  which 
had  properties  characteristic  of  both  the  emission  theory  and  the 
usual  form  of  the  undulatory  theory.  While  lecturing:  in  1911,  he 
proposed  as  an  experimental  test  to  the  theory  that  if  a stream  of 
electrons  had  a strong  beam  of  light  thrown  directly  across  their 
path  slight  deflections  of  the  electrons  might  be  expected.  C.T. 
Knipp^  attempted  the  experiment  in  the  following  year,  using  a 
cathode  beam  twisted  into  a spiral,  by  means  of  a magnetic  field, 
wmcn  fell  on  a photographic  plate  leaving  a trace  in  the  form  of  a 
circle.  Although  much  work  was  done  at  that  time  in  the  laboratory 
by  Knipp,  and  later  by  O.A.  Randolph  (1513)  and  also  C.F.  Hill 
(1915) ; yet  the  difficulties  of  obtaining  high  vacua,  together  with 
the  great  mechanical  complications,  prevented  satisfactory  results 
from  being  obtained. 

Owing  to  the  fact  that  since  that  time  some  prominent  physi- 
cists have  modified  their  views  concerning  the  electromagnetic 


1.  J.J..  Thomsen,  "Electricity  and  Matter" , Ch.O,  pp  50-70;  Phil. 
Mag.  , V.  235,  Feb.  1910. 

3 C.T.  Knipp,  Phys.Rev.  , V.  04,  p 477. 

3.  H.  Bateman,  Phil. Mag.  , V.  350,  p 405,  1917. 

A.  Einstein,  Phys.  Zeitschr. , V.18,  p 131,  1517. 

See  also 

J. H,  Poynting,  Phil. Trans.,  V. 171,  p 377. 

W.  Wei n,  Ann. Phys. Chem. , Ba.  47,  p 027. 

K. A.  Lorentz,  Encyklopadie  der  Mathemat iscnen  Wissenschaf ten, 
Bd.47,  p 327. 

Sir  J.  Larrnor,  Proc. Inst . Congr.  of  Math.,  Cambridge,  V.l,  1313. 
E.  Cunningham,  "The  Principle  of  Relativity",  Ch.  15,  1914. 

D.K.  Mallik,  Phil. Mag. , p 144,  July, 1913. 

H.  Bateman,  Phil. Mag. , Oct. 1913,  han.1914;  "Messenger  of  Mathe- 
matics", May, 1915;  Amer.Journ.  of  Math.,  Apr. 1915. 

W. G.  Brown,  Phil. Mag. , p 283,  Aug. 1315. 


beam  of  Cathode  rays 


5 

Plate  5).  A McLeod  gauge  capable  of  measuring  0.00001  mm.  of  mercu- 
ry pressure  with  a difference  in  level  of  1 mm.  determined  accurate- 
ly tiie  lower  limit  of  tiie  vacuum  while  tne  apparatus  was  in  operation 

‘fne  electron  discharge  chamber  (See  Plate  5,  Fig.l)  was  con- 
structed from  a cylindrical  glass  jar  9.3  cm.  in  diameter  anu  ?6  cm. 
long,  inside  measurements.  Three  holes,  each  3.5  cm.  in  diameter, 
were  drilled  through  the  jar,  one  on  the  bottom  and  one  on  each  side, 
tneir  centers  being  3.5  cm.  and  6.5  cm.,  respectively  from  the  bot- 
tom. A P3O5  bulb  with  ground  joint  connection  was  sealed  into  the 
bottom  with  Bank  of  England  sealing  wax.  The  seat  or  a ground 
joint,  the  plug  of  which  carried  a Wehnelt  Cathode  (See  Plate  3)  was 
sealed  into  the  right  side,  while  a tube  lo  cm.  long  closed  by  a 
quartz  window  at  the  outer  end,  and  silvered  on  tne  inside  was  sealed 
into  the  left  side.  Tne  axes  of  these  two  tubes  were  parallel.  A 
charcoal  bulb  and  also  the  exhaust  tube  branched  from  the  silvered 
tube  near  the  seal.  The  radiation  used  in  the  experiment  was  admit- 
ted through  tne  quartz  window  into  this  tube,  through  which  it  was 
: conducted  into  tne  electron  discharge  chamber.  Six  hundred  and 
forty  turns  of  wo. 13  copper  wire  were  uniformly  distributed  in  two 
layers  over  the  discharge  chamber  in  a space  or  81  cm.  The  radius 
of  the  winding  was  increased  over  tne  wax  seals  by  carrying  tne 
wire  on  a wooden  support,  wnicn  extended  with  tne  coil  over  the  end 
of  tne  chamber.  This  arrangement  gave  ventilation  to  the  wax  seals, 
kept  them  from  melting,  and  also  gave  opportunity  to  view  any  phe- 
nomena  inside  tne  chamber. 

A source  of  electrons  suitable  for  the  experiment  nad  to  be 
developed.  (See  Plate  3),  A small  beam  obtained  from  a large  one 
by  means  of  a platinum  diaphragm  could  not  be  used  because  of  tne 


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» - V.  4 .4  1 | 

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Cathode:  Ray  Exp 
Plate  t\sZ 


ft  Oj  flu/ib 


Quarfz  Plait 
Anode 


Camera 


■Solenoid  binding 


/To Air Pumps' 


/ \ 


n3/; 


Charcoal  Bulb 


Spring  Coupling 


Electron  Discharge  Chamber 


Cathode  Ray  Exp 
P/ote:  ti-3 


8 


presence  cf  a strong  magnetic  field  which  twisted  the  beam  into  a 
spiral.  These  who  had  worked  on  the  experiment  before  had  used  a 
hot  lime  cathode  made  by  placing  a speck  of  sealing  wax  on  a strip 
of  platinum  heated  by  an  electric  current.  This  source  of  elec- 
trons had  two  faults;  first,  it  gave  a very  large  beam  and  second, 
it  was  very  short  lived,  sometimes  lasting  only  a few  seconds.  A 
source,  to  be  successful  for  the  work,  had  to  give  a very  small, 
compact,  permanent  beam  of  electrons. 

A strip  of  platinum  0.5  mm.  wide  was  cleaned  with  nitric  acid 
and  amonium  hydroxide.  A tiny  drop  of  strontium  hydroxide  was 
placed  on  the  strip  after  which  it  was  dried  by  gently  heating  with 
an  electric  current.  After  the  second  application  the  strip  was 
heated  to  500° C to  harden  the  deposit.  A small,  almost  microscopic 
piece  cf  barium  re3inate  was  then  placed  centrally  on  the  spot  and 
the  whole  carefully  heated  so  as-  to  evaporate  the  resin  and  leave 
barium  oxide.  After  two  or  three  coats  cf  barium  oxide  the  strip 
was  glowed  to  cherry  red  for  several  minutes  in  order  to  drive  off 
all  organic  material.  A coating  of  approximately  0,1  mm.  in  diam- 
eter was  thus  obtained  which  gave  an  intense  and  compact  permanent 
beam  cf  electrons  without  the  use  of  a diaphragm. 

The  temperature  at  which  the  organic  material  is  driven  off 
is  very  important.  'Too  high  a temperature  causes  some  sort  of 
chemical  change,  leaving  a dark  deposit  which  does  net  produce  a 
good  electron  beam.  If  the  process  is  carried  on  slowly  under  a 
microscope,  the  heating  current  can  be  regulated  so  as  to  leave  a 
white  deposit  which  is  most  desirable.  A good  beam  has  been  obtains 

6.  C.T.  Knipp,  Phys.Rev. , V. 34,  p 58. 

Nellie  Horner,  Am. Jour n. Science,  p 591,  1513. 


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9 

from  tne  deposit  left  by  one  speck  of  barium  resmate  alone. 

Tne  anoae  consisted  of  a small  piece  or  wire  brought  into  tJae 
discharge  chamber  through  tne  exhaust  tuoe,  ending  at  tne  side  about 
5 cm.  in  front  of  tne  catnode. 

A potential  dinerence  of  about  3OU0  volts  was  maintained  be- 
tween tne  electrodes,  during  tne  operation  of  tne  experiment,  oy 
one  tnousand  small  storage  cells,  These  were  connected  tarougn  two 
water  resistances  and  a paraffine  switch  for  protective  purposes. 

A camera  was  placed  within  tne  discharge  chamber  at  the  end 
opposite  tne  catnode.  Tne  chamber  was  closed  by  a tnick  plate 
glass,  carrying  a winch  connected  to  tne  camera  plate  holder  (See 
Plates  1 and  o).  The  camera  used  was  a small  brass  cylindrical 
box.  (See  Plate  1 and  Fig. 3).  Tne  body  was  81  cm.  in  diameter, 
closed  at  one  end  except  for  a circular  opening  3.8  cm.  in  diameter 
and  3 cm.  off  center  tnrough  which  the  photographic  plate  was  ex- 
posed. a second  cylinder  with  two  sets  of  cross  supports  just  fit- 
ted into  tne  first.  The  plate  holder,  a fiat  brass  disn  ? cm.  in 
diameter,  was  secured  to  a ©haft  which  extended  tnrough  tne  cross 
support  of  tne  second  cylinder,  leaving  tne  nolder  free  to  rotate. 
The  photograpnic  plate  was  stuck  to  the  nolder  witn  half  and  half 
wax,  after  which  it  was  made  circular  with  a diamond  glass  cutter. 
When  the  parts  were  assembled,  the  photographic  plate  was  pressed 
snugly  against  tne  face  of  tne  camera  with  only  a small  portion  ex- 
posed. The  shutter,  a disn  of  aluminum  with  a hole  in  one  9ide  2.8 
cm.  in  diameter  and  2 cm.  off  center,  so  tnat  it  exactly  matched 
the  hole  in  the  face  of  the  camera,  was  mounted  inside  a cap  which 
fitted  over  the  face  of  the  camera.  This  cap  carried  a system  of 

levers  which  held  the  shutter  in  place,  except  when  it  was  released 


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13 

by  a Biagnet  on  the  outside  of  the  discharge  chamber,  acting  on  a 
small  piece  cf  iron  attached  to  one  of  the  levers.  The  shutter  was 
given  mechanical  motion  by  a double  spring  which  rotated  it  once 
around  each  time  it  was  tripped.  A willemite  phosphorescent  screen 
was  deposited  on  the  shutter  in  such  a position  that  it  was  exposed 
when  the  shutter  was  at  rest.  This  enabled  the  operator  to  see  the 
configuration  that  would  be  gotten  on  the  plate  as  a picture, b ef ore 
the  picture  was  taken.  After  a little  experience  the  operator  could 
adjust  the  image  on  the  screen  to  any  desired  size  by  slightly  ro- 
tating the  cathode  and  regulating  the  current  through  the  solenoid. 

Several  kinds  of  photographic  plates  were  tried.  The  electron 
sensitivity  of  a photographic  plate  appears  to  be  in  no  way  con- 
nected 'with  the  light  sensitivity.  The  plates  finally  adopted  were 
Imperial  (special)  Lantern  Plates,  manufactured  by  "The  Imperial 
Dry  Plate  Co.  Ltd. , Crickelwood,  London.  They  had  an  exceedingly 
smooth  gelatine  surface  and  a lew  sensitivity  to  light. 

Two  baffling  plates  were  equally  spaced  in  the  discharge 
chamber  between  the  cathode  and  the  camera  in  order  to  shut  off  any 
stray  light  effects  which  might  darken  the  plates.  The  holes  cut 
through  these  plates  to  allow  the  spiral  beam  to  pass  were  2,5  cm. 
in  diameter. 

Two  sources  of  electromagnetic  radiation  were  used;  a ninety 
degree  carbon  arc,  and  a Ccolidge  X-Ray  tube  (See  Plate  4).  The 
arc  using  white  flame  carbons  and  30  amperes  current  was  placed  in- 
side a light-tight  box  33  cm.  from  the  quartz  window  at  the  end  of 
the  silvered  tube.  The  arc  was  then  about  43  cm.  from  the  beam  of 
electrons  upon  which  it  was  to  fall.  No  lenses  were  used  in  most 
of  the  work,  hence,  a very  intense  beam  of  radiation  rich  in  ultra 


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' 

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4 


Hu 


wyv- 


llo  V.D.C.  Jolenoid  Circuit 


t ySolenoid  Hi  riding] 


4/o  Its  St.Q. 


it 1 


Note:  llo  l/olts  A C used  for 
Pump  Motors,  Heating 
Coils  etc . 

Pcneru 


'Shutter  Circuit 


Zo/olts 


Wiring  Layout 

Cathode  Ray  Lxp 

Plate  /Ve4. 


14 


violet  reached  the  discharge  chamber. 

The  X-Ray  tube  used  was  the  Universal  Type  Coolidgs  tubs  with 
a broad  focal  spot.  This  was  excited  by  a 6 inch  spark  Klingelfuss 
induction  coil  operated  by  a Wehnelt  interrupter  on  110  volts  P. C. 
This  tube  was  mounted  inside  a heavy  lead  box  so  that  the  target  was 
21  cm.  frojp  the  quarts  window  and  3?  cm.  from  the  beam  of  electrons. 

Ill  OPERATING  CONDITIONS 

Pus  to  the  fact  that  the  experimental  operations  of  this  re- 
search were  very  critical,  the  exact  conditions  under  which  the  re- 
sults were  obtained  are  definitely  stated.  The  vacuum  was  always 
0.00001  mm.  of  mercury  or  less  when  the  exposure  was  started.  At 
the  end  of  a series  of  exposures  the  pressure  was  measured  and  it 
was  seldom  higher  than  0,00001  mm.  The  discharge  chamber  was  freed 
cf  water  and  mercury  vapors  by  a'P30g  bulb,  a large  cocoanut  char- 
coal bulb,  and  a liquid  air  trap,  tne  last  two  being  immersed  in 
liquid  air  (See  Plate  5).  Liquid  air  was  never  applied  until  the 
pressure  was  0.00003  mm.  cf  mercury,  sc  that  the  absorbing  capacity 
cf  the  charcoal  was  saved  to  remove  any  gases  given  off  by  the  hot 
cathode  while  photographs  were  being  taken. 

The  Wehnelt  cathode  was  heated  to  a degree  of  hoc ness  gained 
by  experience  until  a beam  cf  electrons  of  sufficient  intensity  was 
obtained  to  make  an  impression  on  the  photographic  plate.  Richard- 

7 

son  has  shown  that  the  number  cf  electrons  emitted  from  a hot  body 
is  a function  of  the  temperature,  the  emission  current  being  given 

?.  O.W,  Richardson,  "Emission  of  Electricity  from  Rot  Bodies'', 

Chap. 2,  pp  32  and  SO;  Phil.  Trans.,  A. ,V,20i,  p 530. 


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15 

by  the  following  formula: 

I = CT2e 

when  0 and.  d are  constants,  T is  tne  absolute  temperature,  and  e Is 
tne  natural  logaritnmic  base.  Their  velocity,  however,  depends 
largely  upon  the  potential  gradient  through  thicn  tney  fall®.  A 
very  low  voltage  acting  against  the  cathode  will  prevent  tne  escape 

Q 

of  electrons,''  even  though  the  cathode  may  oe  at  a very  nigh  tem- 
perature. Four  degrees  of  hotness  were  recognized  and  recorded. 
They  were  cnerry  red,  hot,  very  hot,  and  white  neat,  the  correa- 
pondmg  temperatures  being  approximately  770,  SOO,  1150  and  1550 
degrees  centigrade  respectively.  Because  of  tne  high  vacuum  used 
and  the  absence  of  any  track  of  mercury  vapor  it  was  sometimes  very 
difficult  to  start  tne  discharge  even  cn  the  application  of  2000 
volts.  It  could,  however,  usually  be  induced  to  start  by  heating 
the  cathode  very  hot  for  an  instant.  When  once  tne  beam  was  start- 
ed, it  invariably  started  readily  thereafter  and  at  lower  poten- 
tials.*^ It  was  found  that  a trace  of  mercury  vapor  caused  the 
discharge  to  start  very  easily.  After  the  beam  was  started  the 
cathode  was  rotated  until  it  was  projected  against  the  side  of  tne 
tube.  When  tne  current  was  turned  on  m the  solenoid  circuit 
around  the  discharge  chamber,  tne  beam  was  caugnt  m a magnetic 
field  of  approximately  ISO  gausses  ana  wound  into  a spiral  which 


8.  O.W.  Richardson,  "Emission  of  Electricity  from  not  Bodies",  Ch. 5, 

p. 15b, 

9.  O.W.  Richardson,  "Emission  of  Electricity  from  Rot  Bodies" ,Ch. 5, 

pp. 169-171. 

10.  Nellie  N.  Horner,  Am. Jcurn. Sci. , V.33,  p 596,  Dec. 1913. 


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IS 


traversed  the  length  of  the  discharge  chamber,  striking  on  the 
willemite  screen  on  the  outside  of  the  shutter  (See  Plates  1 and  4). 
The  phosphorescent  spot  was  moved  by  means  of  a focusing  magnet 
placed  on  the  outside  of  the  discharge  chamber  until  it  was  central- 
ly located  on  the  screen  before  exposures  were  made. 

The  electron  beam  from  the  cathode  could  be  easily  seen  in 
spiral  form  within  the  discharge  chamber  for  pressures  in  the  neigh- 
borhood of  0.001  mm.  of  mercury.  The  pitch  and  diameter  of  the 
spiral  could  be  changed  at  will  by  rotating  the  cathode  and  regulat- 
ing the  solenoid  current.  With  pressures  of  0.0001  mm.  and  lower 
the  beam  couid  no  longer  be  seen  and  only  an  estimate  could  be  made 
concerning  the  pitch  of  the  spiral.  While  the  electron  beam  was 
passing  in  front  of  the  tube  through  which  the  radiation  entered  it 
was  subjected  to  any  effect  the  radiation  might  have  upon  it.  The 
light  radiation  in  the  form  of  a beam  2.3  cm.  in  diameter  was  throwr 
at  an  angle  of  90°  acrbss  the  path  of  the  electron  beam,  hence,  any 
action  on  the  electrons,  due  to  the  radiation,  took  place  during 
the  time  the  electrons  were  passing  through  a space  of  about  2.2  cm, 

When  X-rays  were  used,  due  to  the  size  of  the  slit  in  the  lead 
box  around  the  Coolidge  tube,  the  space  filled  with  radiation 
through  which  the  electrons  passed  was  1.2  cm.  Nc  radiation  was 
permitted  to  fall  on  the  Wehnelt  cathode. 

The  magnet  operating  the  camera  shutter,  being  only  13  cm. 
away  from  the  photographic  plate,  had  a small  displacement  effect 
on  the  electron  beam  at  tne  instant  the  shutter  was  tripped.  The 
revolving  s nut ter,  however,  had  a time  lag  of  about  0.2  second 
between  the  time  it  was  tripped  and  the  time  it  opened  to  take  the 
photograph.  The  shutter  magnet  current  was  operated  by  a tapping 


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17 


key  which  was  never  closed  for  prooably  more  than  0.01  of  a second. 
Tnis  gave  ample  time  for  any  displacement  effect  on  tne  Dean,  to  dis- 
appear before  tne  picture  was  taken. 

IV  PHOTOGRAPHIC  MANIPULATION 

When  tne  camera  had  been  placed  in  position  on  tne  insiae  of 
tne  discharge  chamber  ana  the  vacuum  brought  to  tne  proper  point, 
tne  catnode  was  neatea;  tne  solenoid  current  turned  on;  and  tne  dis- 
charge potential  placed  across  tne  electrodes.  At  urst,  a faint 
phosphorescent  trace  appeared  on  tne  screen,  wnicn  rapidly  increased 
in  intensity  until  a circle  or  an  arc  of  a circle  was  visible.  The 
intensity,  size,  snape  ana  position  of  the  phosphorescent  spot  could 
then  oe  changed  by  adjusting  or  regulating  tne  pitch  of  tne  cathode 
ray  spiral,  the  temperature  of  the  hot  catncue  ana  tne  solenoid  cur- 
rent. The  focusing  coil  enabled  tne  final  adjustment  to  be  made, 
after  which  a succession  of  photographs  were  taken.  This  process, 
wnicn  ordinarily  took  several  minutes,  usually  caused  a let  down  in 
tne  vacuum,  due  to  tne  continued  heating  or  tne  platinum  strip,  of 
a few  hundred  thousandths  of  a millimeter.  The  vacuum,  however,  was 
quickly  restored  to  below  0. 00001  mm.  oy  tne  rapid  acting  pumps. 
After  everything  was  in  readiness  the  source  of  radiation  was  start- 
ed and  the  photographing  began.  Six  phctcgrapns  were  taken  on  each 
plate.  A practice  was  made  oi  taking  tne  odd  numoered  pictures 
without,  and  tne  even  numbered  ones  witn  tne  radiation  falling  on 
tne  electron  spiral.  Between  exposures  tne  screen  controlling  the 
radiation  had  to  be  operated  ana  tne  photographic  plate  turned  for- 
ward to  its  next  position.  The  time  between  pictures  was  5 to  b 
seconds.  The  average  time  elapsing  between  the  first  and  the  sixth 


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18 

exposure  was  87  seconds. 

V MEASUREMENTS 

After  the  photographic  plates  had  been  developed  and  numbered, 
they  were  carefully  examined  to  see  which  could  be  subjected 

to  measurements.  A plate,  tc  be  of  value  for  measuring,  had  to  pos- 
sess certain  qualifications  adopted  as  standard.  First,  tne  elec- 
tron trace  had  to  be  of  sufficient  intensity  to  be  easily  seen  with 
the  naked  eye,  since  faint  traces  could  not  be  seen  at  all  under 
the  microscope  used  in  measuring  tne  photographs.  All  of  the  plates 
were  under-exposed,  hence,  tne  development  nad  to  be  forced,  result- 
ing in  many  discolored  plates.  Second,  the  trace  had  to  form  an  arc 
of  a circle  of  sufficient  length  to  measure  its  diameter.  Third, 
the  edges  of  the  circle  had  to  be  snarp  so  tnat  the  error  of  measure- 
ment might  be  small.  Traces  that  faded  out  along  tne  edges  were  of 
no  value  because  no  marks  could  be  found  on  which  to  set  tne  measur- 
ing instrument.  Fourth,  tne  six  pictures  on  a plate  had  to  be  simi- 
lar so  tnat  the  same  measurement  could  be  taken  on  each  one. 

After  the  plate  had  been  selected,  a very  fine  line  was  drawn 
across  each  circle  to  indicate  the  diameters  to  be  measured.  This 
diameter  was  measured  by  means  of  a small  dividing  engine  (See  Fig. 

3)  tne  screw  of  which  was  graduated  to  0.001  cm.  This  screw  was 
mounted  in  a rigid  frame  holding  small  strips  of  plate  glass  upon 
which  the  photographic  plate  was  placed  in  sucn  a way  that  light 
could  be  reflected  through  it.  A needle  point,  ground  to  look 
sharp  under  twenty  diameters  magnification,  and  mounted  on  tne  car- 
riage, was  set  alternately  on  tne  edges  of  tne  photographic  circle 
at  the  ends  of  a given  diameter  and  the  readings  of  the  micrometer 


. 

» 

. ' 

. 

. 

. 

. 


. 

. 


Cl- 


so 

taken.  The  difference  between  readings  gave  the  diameter.  A binoc- 
ular microscope  of  low  power  was  used  to  set  the  needle  point  accu- 
rately. 

VI  RESULTS 

A table  has  been  prepared  showing  quantitatively  for  each 
plate  tne  four  experimental  quantities  which  affected  the  pictures 
most;  the  electrode  voltage,  the  solenoid  current,  the  cathode  con- 
dition and  the  vacuum.  The  condition  of  the  plate  after  it  was 
developed  and  the  effects  evident  on  it  are  also  shown  in  the  table. 
Consecutive  plates  have  been  numbered  10,  20,  30,  40,  etc. , because 
there  are  supposed  to  be  six  pictures  on  each  plate.  On  the  fourth 
plate,  then,  tne  third  picture  would  be  referred  to  as  43,  the 
sixth  as  46,  etc.  Due  to  lack  of  3pace  in  a table  containing  so 
many  columns  abbreviations  have  been  used  in  many  cases;  such  as, 
st.  for  stained,  F for  fogged,  ft.  for  faint,  C. Ip.  for  traces  im- 
perfect, e.p.  for  edges  poor,  exp.  for  experimental  plate,  and  pos, 
for  positive. 

When  the  even  numbered  circles  - those  taken  with  tne  radi- 
ation turned  on  and  numbered  2,4,6  - have  a smaller  average  diam- 
eter than  the  odd  numbered  one3j  those  taken  while  radiation  was  off 
the  effect  is  defined  as  positive.  If  the  tendency  seems  clearly 
to  be  in  that  direction,  but  the  results  are  not  absolute,  the  re- 
sult is  designated  as  "positive?". 

Another  effect  is  undouotedly  present,  and  that  is  a scatter- 
ing of  the  electrons  or  a diffusion  of  the  electron  beam.  This  ef- 
fect can  be  noted  even  when  the  traces  are  not  circles.  When  it  is 
present  the  table  indicates  tne  fact  by  "yes"  in  the  last  column. 


31 

Plate 

Condi- 

Elec- 

Solenoid 

Cathode 

Vacuum 

Eft'  ect 

Scatter- 

t icn 

trode 

Current 

Condi- 

in 

jng? 

No. 

of 

Volt- 

in 

t i on 

mm. 

Plat  e 

age 

amperes 

Radia- 

fcion  - 

Carbon  used  with  condenser  lens  and 

glass  plate 

340 

Good 

3000 

15.0 

hot 

B0. 00002 

Pos.  ? 

Yes? 

350 

Exp. 

360 

Good 

1935 

12.5 

V.  H. 

B0. 00003 
EO. 0003 

Pcs.  ? 

No  | 

3?0 

Good 

3070 

13.0 

V.  H. 

BO. 00002 
EO. 0002 

Pcs. 

Yes? 

360 

Good 

1900 

13.0 

V.H. 

BO. 00003 

Pos.  ? 

Yes 

390 

Good 

1900 

13.4 

Whit  e 

BO. 00002 

Neg.  ? 

Yes? 

Radiation  - 

Carbon  arc 

used  with  glass  plate 

o 

o 

•* 

Good 

2000 

15.0 

V.H. 

BO. 00001 
EO. 0001 

Neut . 

Yes? 

i 410 

Good 

1925 

13.8 

hot 

BO. 00001 

Neg. 

No 

Radiation  - 

Carbon  arc 

used  with  quartz 

plats 

430 

E,  P.  C. Ip 

430 

Good 

1950 

18.0 

hot 

BO. 00003 
EO. 0002 

N eut . 

Yes 

440 

Good 

1975 

15.7 

hot 

BO. 00003 

Neut . 

Yes 

450 

Good 

1900 

13.6 

hot 

BO. 0002 
EO. 0003 

Pos.  ? 

Yes 

460 

Good 

1935 

13.0 

V.H. 

BO. 00006 
EO. 0001 

Neut. 

Yes? 

470 

C.  Ip. 

480 

Good 

1940 

11.6 

V.H. 

BO. 00001 
EO. 0001 

N eut . 

No 

490 

Good 

2075 

16.6 

hot 

BO. 00001 

Neut . 

No 

500 

C.  Ip  E.P 

• 

510 

St.C. Ip. 

520 

Ft .E.P. 

\ 


Plate 

No. 

Condi-  Elec- 
tion trode 

of  Volt- 

Plate  age 

Solenoid 

Current 

in 

amperes 

Cathode 

Condi- 

tion 

Vacuum 

in 

mm. 

Effect 

22 

Scatter- 

ing? 

X-R&ye 

- First 

Series 

530 

Good  3000 

15.5 

hot 

BO. 00001 

Pos. 

Yes 

540 

C Ip.E.P. 

550 

F. St.C.Ip 

Yes 

560 

C. Ip.E.P. 

Yes 

570 

Broken 

Yes 

580 

Good  3100 

26.4 

hot 

BO. 00001 

Pcs . 

Yes 

530 

C.  Ip. E.P. 

Yes 

600 

C.  Ip. F. St. 

610 

C.  Ip.  E.P. 

Yes? 

X-Rays 

- Second 

Series 

700 

Ft. 

710 

Good  1800 

15.0 

hot 

EO. 00001 

Pos. 

"V  o o 

730 

Good  1900 

16.3 

V.H. 

BO. 00001 

Pcs. 

Yes 

730 

Good  3100 

16. 5 

V.H. 

BO. 00001 

Pos. 

Yes 

740 

Nothing 

750 

C. Ip.E.P. 2000 

17.0 

V.H. 

BO. 00001 

Pcs. 

760 

Good  3000 

18.8 

V.H. 

BO. 00001 

Pcs. 

Yes? 

770 

C. Ip. E.P.  2050 

16.0 

V.H. 

BO. 00U01 

Pos.  ? 

780 

Good  3000 

30.5 

V.H. 

BO. 00001 

Pos. 

Yes 

790 

Good  2000 

16.0 

hot 

BO. 00003 

Pos . 

Yes? 

33  | 

If  the  tendency  seems  clearly  to  be  in  tnat  direction,  but  if  the 
indications  are  not  absolute,  tne  result  is  designated  as  "yes?". 

Thirty  three  plates  were  exposed  before  the  apparatus  was 
brought  under  control,  and  one  was  obtained  that  could  be  measured. 
Of  the  next  twenty  eight,  however,  fifteen  were  perfect  enough  to 
measure  and  five  others  were  examined  for  a scattering  effect. 

Four  types  of  radiation  were  used  in  this  series  of  plates, 
hence  they  will  be  divided  into  groups  depending  on  the  radiation. 
The  carbon  arc  wa3  used  in  tne  first  tnree  groups  with  plates  340 
to  390  inclusive.  A condensing  lens  m3  used  with  it  and  a plate 
glass  window  covered  tne  end  of  the  tube  that  conducted  tne  radi- 
ation into  tne  discharge  chamber.  Plates  400  and  410  were  taken 
with  tne  lens  removed.  For  the  next  group  including  430  to  490  a 
quartz  plate  was  substituted  for  tne  plate  glass  window.  This  per- 
mitted a beam  of  light,  rich  in  ultra  violet,  to  act  on  tne  elec- 
tron beam.  Hard  X-Ray3  were  used  with  plates  50  to  61  inclusive. 

Plate  wo. 610  ending  tne  last  series  was  taken  Aug. 11,  1921. 

No  more  were  attempted  until  tne  Christmas  vacation  1931-1923.  In 
the  meantime  the  first  series  had  been  examined  and  marked  effects 
had  been  found  on  tne  plates. 

At  the  time  tne  photographs  were  measured,  the  data  taken  was 
put  into  graphical  form  in  order  that  it  might  be  more  easily  inter- 
preted. The  logical  way  to  plot  the  results  would  be  to  plot  time 
as  abscissae  against  the  diameter  of  the  measured  circles,  the  time 
starting  when  the  first  exposure  on  the  plate  was  made.  The  time 
between  exposures  was  noted  for  tne  plates  370  to  440  inclusive. 

The  variation  was  so  slight  that  it  was  not  thought  necessary  to 
have  an  extra  observer  simply  for  tne  purpose  of  noting  time.  For 


. 


. 


. 


. 


. 

. 

. 

. 

♦ 

24 

this  reason  the  abscissas  used  were  simply  the  exposure  numbers  as 
they  occurred  on  the  plate. 

The  curves  for  plates  370,  440,  480  and  490  were  chosen  as 
representative  of  all  the  results  that  were  obtained  when  the  carbon 
arc  was  used  as  a source  of  radiation.  (See  curve  sheets  for  above 
plates).  The  slope  of  the  curve  is  mainly  due  to  the  fact  that  the 
solenoid  current  slightly  decreased  as  the  resistance  of  the  coil 
increased  by  heating.  Changes  in  elope  such  as  occur  in  Nol440  and 
490  are  probably  due  to  slight  variation  in  voltage  a3  a 135  volt 
D. C.  line  supplied  the  current. 

Plate  No, 370  apparently  gives  results  that  are  distinctly  posi- 
tive. This  is  the  only  one,  however,  as  noted  above,  of  the  thir- 
teen examined  of  the  series  taken  when  arc  light  radiation  fell  on 
the  electron  beam  that  snows  a decided  positive  result.  Four  were 
"positive"  but  the  magnitude  of  the  results  were  within  the  experi- 
mental error;  six  were  neutral;  one  was  slightly  and  another  dis- 
tinctly negative. 

After  measuring  and  examining  this  series  of  plates,  the  fol- 
lowing may  be  said.  So  far  as  the  positive  effect  is  concerned 
the  only  conclusion  admissible  here  is  that  under  the  conditions  of 
the  experiment,  if  radiation  of  wavelengths  from  8000  to  13000 
Angstrom  units  falls  across  a stream  of  rapidly  moving  electrons, 
there  may  be  a slight  positive  effect  which  is  possibly  less  than 
the  experimental  error. 

Concerning  the  scattering,  the  following  was  found:  four  were 
neutral;  five  showed  slight  indication  of  scattering,  and  four 
clearly  showed  a scattering  effect.  There  seems,  then,  to  be  evi- 
dence that  a scattering  effect  was  present. 


' 


. 

. 


The  first  X-Ray  plate  (when  X-Rays  fell  across  tne  electron 
beam)  measured,  No.5o0,  (See  Curve  sneet,  530  and  Fig. 4)  gave  both 

a decided  positive  effect  and  a scattering.  Picture  No. 531  was  com- 

> 

pletely  darkened  by  togging,  hence  for  this  plate  tnere  are  only 
five  points  on  the  graph  sneets.  The  remainder  of  tne  series  were 
carefully  examined,  but  only  one  was  found.  No. 580,  (See  Curve 
sneets  580  and  Fig. 5)  that  would  subject  itself  to  measurement.  This 
also  gave  large  positive  results. 

The  last  X-Ray  series,  700  to  730,  (the  Nos.  b20  to  630  were 
omitted)  was  taken  to  cneck  tne  previous  worx.  In  order  that  each 
plate  might  be  brought  to  account  and  excuses  might  not  nave  to  be 
made  for  missing  plates,  the  greatest  care  was  taken  in  making  the 
exposures.  This,  however,  was  too  much  to  expect  of  apparatus  so 
difficult  to  manipulate.  Plate  700  was  almost  a blank.  No. 740  was 
a complete  blank,  while  tne  eages  were  so  poor  and  the  traces  so 
indistinct  on  No. 750  and  770  that  they  could  be  inspected  only,  and 
not  measured.  Since  Plate  710  was  too  faint  to  measure  under  the 
microscope,  a needle  point  divider  was  used.  The  error  in  measure- 
ment was  hign,  which  accounts  for  the  divergence  of  tne  curves  (See 
Curve  sneet  710).  The  two  effects,  however,  were  plainly  visiDle 
to  the  naked  eye. 

Plate  720  snows  a large  positive  effect.  The  probable  error 
here  is  +0,008  cm.  (See  Curve  sneet  720  ana  Fig. 6).  An  examination 
of  Fig. 6 will  8hov?  tnat  tne  effect  is  easily  visible  to  tne  eye. 

Plate  7o0  (See  Curve  sneet  730  and  Fig. 7)  nas  tne  largest 
positive  effect  obtained.  An  examination  of  tne  data  in  tne  table 
shows  tnat  all  conditions  here  were  favorable  for  very  hign  velo- 
city electrons  projected  at  a large  angle  witn  tne  axis  of  tne 


26 

solenoid.  Hence  we  would  expect  a large  effect, 

Plate  760  was  measured  along  a radial  diameter,  and  also 
along  a diameter  at  ngnt  angles.  (See  Curve  sheet  760).  Tne  last 
two  curves  form  a large  angle  with  the  first.  This  fact  can  only 
be  explained  on  tne  assumption  tnat  tne  velocity  distrioution  in- 
creased as  the  plate  was  being  exposed.  Both  sets  of  curves  show  a 
positive  effect. 

Plate  770,  although  imperfect,  was  measured  as  carefully  as 
possible.  The  results,  although  inconsistent,  show  a tendency  to- 
ward a positive  effect.  No  curve  was  plotted  for  this  plate. 

Plate  780  shows  a positive  effect,  larger  tnan  tne  experi- 
mental errors,  although  not  so  large  as  the  otners.  ^See  Curve 
sheet  780). 

On  the  last  plate.  No. 790,  the  last  two  pictures  are  entirely 
different  from  the  ethers.  Consequently,  no  direct  comparison  can  be 
made.  Pictures  792  and  794  are  distinctly  smaller  tnan  791  and  796 
This  shows  a positive  effect. 

From  the  series  just  presented  we  see  tnat  from  ten  plates 
examined,  all  but  one  snowed  a distinctly  positive  errect.  This 
single  plate  had  a positive  tendency  but  was  too  imperfect  to  ex- 
amine accurately.  Thirteen  were  examined  for  scattering.  Ten 
showed  distinct  scattering  and  three  were  inclined  in  tnat  direct io^. 

We  may  conclude,  tnen,  tnat  under  the  conditions  of  the  ex- 
periment, X-Hays  cend  to  decrease  tne  velocity  of  an  electron  beam 
wnen  tnrown  across  tneir  patn;  also  tnat  tne  electron  beam  is  dif- 
fused or  scattered  by  the  X-Pay3. 


. 

. 


w . . 


. 

. 

. 


3? 


Plate  No.  530 


Fig.  4 

This  photograph  snows  *the  traces  made  'by  tne  electron  beam  when 
it  was  alternately  exposed  to  hard  X-rays  (No. 2,4,6  exposed).  Elec- 
trode voltage  3000  volts,  solenoid  current  15.5  ampere,  vacuum 
0.00001  mm.  mercury.  This  plate  shows  the  positive  effect  and  also 
fja  scattering.  See  Curve  Sheet  No.  530. 


■ • i 

. 


• . 


Plate  No,  580 


Fig.  5 

This  photographs  shows  the  traces  made  by  the  electron  beam 
when  it  was  alternately  exposed  to  hard  X-rays  (No. 3,  4,  6 exposed). 
Electrode  voltage  2100  volts,  solenoid  current  26.4  amp.,  vacuum 
0.00001  mm.  of  mercury.  See  curve  sheet  No. 580.  This  Plate  shows 
|a  positive  effect  and  also  a scattering.  Here  the  different  diam- 
eters of  the  circles  can  be  noted. 


22 


T31  o 


ate  No.  720 


Fig.  6 

This  photograph  was  in  one  taken  in  the  second  series  for  check-} 
ing  purposes.  Traces  No. 2,  4 and  6 were  made  by  electrons  which  had 
been  exposed  to  hard  X-rays.  Cathode  voltage  1900  volts,  solenoid 
current  16.2  amperes,  vacuum  0.00001  mm.  mercury.  This  photograph 
shows  distinctly  that  the  electrons  moving  in  the  spiral  when  ex- 
posed to  hard  X-rays  as  described  in  the  text,  are  appreciably  slewed 
down  in  velocity  and  hence  under  the  strong  magnetic  field  are  twist- 
ed into  a spiral  of  smaller  diameter.  This  is  clearly  shown  by  the 
traces  2,  4,  6 which  are  smaller  in  diameter  than  traces  1,  3,  5 
which  ?/ere  not  exposed  to  X-rays.  The  scattering  effect  is  also 
present . 


. 

. 

. 


. 


30 


Plate  No.  730 


Fig. 7 

This  photograph  snows  the  san  e as  No.  730,  Fig.  6.  It  was  taken 
under  slightly  different  conditions.  Cathode  voltage  3100  volts, 
solenoid  current  16.5  amp.,  vacuum  Q . 00001  mm.  mercury. 


Piute,  yw 


Z 


34 


Plate  P90 

mo 


two 


lift) 


X /?ay  Pkte  530 


l%D 


S 


iViO 


mo 


1200 


IW 


im 


Picture  Y)  umbers. 


3 1 


S 


36 


'± 


40a 


41  * 

VII  DISCUSSION 

The  examination  of  the  figures  4,  5,  6 and  7 raises  a number  of 
questions,  some  of  which  must  for  the  present  remain  without  satis- 
factory answers.  Why  does  the  electron  beam  "Spiral"  down  the  dis- 
charge chamber?  Why  is  there  a continuous,  almost  circular  trace, 
on  the  photographic  plate  since  one  would  expect  that  a plane  cross 
section  of  a spiral  would  give  a point  and  not  what  is  apparently  a 
projection  of  a spiral?  Why  is  the  trace  sharp  and  narrow  on  one 
side  while  it  is  wide  and  diffused  on  the  other?  Why  is  it  not  a 
circle?  The  following  are  answers  to  the  above. 

Consider  in  Fig. 8 the  vector  OA  to  lie  parallel  to  the  axis  of 

the  discharge  chamber.  If  an  electron 
beam  were  projected  along  OB,  its  ve- 
locity could  be  resolved  into  the  twc 
components  OC  and  OA,  where  OC  is  at 
right  angles  to  the  magnetic  field. 

The  latter  component  would  be  convert e 
into  a circle, the  radius  cf  which  we 
Fig. 8 might  compute  if  we  knew  the  original 

velocity,  or,  if  we  know  the  radius  we  can  find  the  velocity  from 
the  equations 


or 


F = Hil  = Hev  = ma  = 


He 


mv* 


wnere  H is  the  intensity  of  the  magnetic  field,  v is  the  velocity  cf 
the  electrons  in  the  beam,  m is  the  mass  of  the  electron,  _e  is  its 
charge,  and  r_  is  the  radius  of  the  circle  into  which  the  beam  is 

changed. 


■ 

. 

■ 


. 


'I 

. 


43 


The  component  OA  is  unchanged,  and  our  components  are  now  like  Fig.  9 


B 


/ \ 
/ 1 

V I 

\ / 
'X 

./  N 


Hence,  the  resultant  motion  is  a 
spiral  along  OBA,  which  travels 
down  the  tube  at  a velocity  OA. 

Consider  the  original  beam 
OB,  Fig. 8,  as  it  comes  from  the 


11 


Fig.  9 

oxide  on  the  heated  platinum.  The  velocities  of  the  electrons  in 
the  beam  are  distributed  according  to  Maxwell's  distribution  law. 

The  electrons  at  this  point  are  caught  in  the  intense  electric  field 
and  all  are  accelerated  through  the  same  change  in  velocity.  This 
leaves  the  velocity  difference  between  the  slowest  and  the  fastest 
the  same  as  it  was  before  the  acceleration.  Different  velocities 

are  twisted  into  circles  of  different 
radii  by  the  magnetic  field,  and  since  all 
the  electrons  start  at  one  spdt  and  initial 
ly  have  the  same  direction,  the  traces  are 
all  tangent  tc  the  point  A,  Fig. 10,  which 
corresponds  to  the  emission  point, 
equation  we  see  that  the  radius  of  a circle 
into  which  a beam  is  twisted  is  directly  proportional  to  tne  vector 
velocity  OC,  # 


B 

i \ 

» / 

‘ i 
\ / 


/ \ 

< » 

y 

✓ \ k 


0-‘ 


\ ■ 
l I 

x 


i 


\ 

i 

\ i 

\ i 

\ I 
\ / 
/\ 

✓ v 


/ » 

' ! 


D B 


Fig.  11 


11.  O.W.  Richardson,  "Emission  of  Electricity  from  Hot  Bodies", 

Ch.5,p  139-178,  see  p. 161  (1916);  Phil.  Trans.  A. ,V. 301 ,p  503, 
(1903);  Phil. Mag;.  .Vcl.  18.p695  (1305). 


43 


If  we  consider  the  spiral  in  Fig.  11  we  may  assume  that  if  a 
group  of  electrons  started  from  the  source  at  0 at  the  same  instant, 
the  fastest  of  them  would  reach  the  plate  A in  a given  time  t.  Some 
a little  slower  would  have  arrived  at  B and  would  still  have  one 
half  turn  to  make  before  striking  the  plate.  This  would  bring  them 
to  a point  B on  the  trace  in  Fig. 10.  Otner  electrons  with  a still 
smaller  velocity,  being  af  the  points  C and  C,  Fig.}l,  would  fall  at 
A and  D respectively.  Fig. 10. 

This  explanation  accounts  entirely  for  tne  sharp  edge  at  A and 
the  wide  and  diffused  part  between  B and  D.  If  we  also  consider 
that  the  outside  of  the  trace  is  an  envelope,  tangent  to  arcs  of  a 
family  of  circles  having  a common  point  A,  we  can  also  readily  see 
why  the  trace  is  not  a perfect  circle. 

A possible  effect  cn  the  results  obtained  might  come  from  the 
action  of  the  radiation  on  tne  apparatus  itself.  A study  of  Plate 
4 will  show  that  the  radiation  after  passing  tne  electron  beam  would 
strike  the  side  of  the  discharge  chamber,  freeing  some  electrons. 

A positive  charge  would  be  built  up  on  the  glass  across  from  the 
anode  until  the  potential  reacned  a point  such  as  to  prevent  tne 
escape  of  any  mere  electrons.  This  charge  would  leak  off,  in  part 
at  least,  during  the  time  a photograph  was  being  taken  when  the 
radiation  was  turned  off.  If  Crookes  dark  space  is  small  compared, 
to  the  space  between  the  cathode  and  the  positive  charge  on  the 
glass  then  an  increase  in  the  velocity  of  the  electrons  would  re- 
sult when  radiation  was  present,  hence,  an  increased  diameter  of 
the  trace  would  be  expected  instead  of  a decreased  one,  as  was 
found. 

No  formal  attempt  will  be  made  to  explain  from  a theoretical 


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44 


point  of  view  the  results  obtained  in  this  research.  It  seems,  how- 
ever, that  it  would  not  be  out  of  place  to  suggest  possible  lines 
along  which  explanations  might  be  found.  As  was  stated  in  the 
introduction,  the  experiment  grew  out  of  a remark  made  by  J.  J. 
Thomson.  He  suggested  that  if  a diffused  pattern  in  the  electron 
trace  was  found  when  radiation  was  thrown  across  the  path  of  the 
electrons,  the  result  might  be  taken  as  indicative  of  the  correct- 
ness of  a theory  of  light  which  he  had  advanced.  ^ C.T.  Knipp,  who 
was  a student  with  Thomson  at  the  time,  saw  the  possibilities  of 
such  a research  and  soon  after  his  return  to  Illinois,  designed  and 
built  the  apparatus  with  which  the  early  work  was  done. 

It  seems  possible  that  if  an  electron  were  projected  at  an 
angle  to  the  axis  of  the  discharge  chamber,  through  electromagnetic 
waves  as  they  are  considered  in  the  usual  form  of  the  undulatofy 
theory,  the  electron  would  be  set  in  a swaying  motion  as  it  advanced 
through  electric  and  magnetic  fields  which  periodically  reversed  in 
direction.  If  the  fields  should  suddenly  cease  to  exist,  the  elec- 
tron would  continue  in  a line  tangent  to  the  path  of  its  motion. 

This  path  in  all  probability,  would  be  at  an  angle  to  the  line  of 
flight  when  it  entered  the  field.  It  seems  improbable,  however,  due 
to  diffraction  and  scattering  effects,  that  the  electric  and  mag- 
netic fields  of  radiation  have  a sharp  well  defined  boundary.  On 
the  contrary,  it  seems  more  likely  that  the  fields  diminish  gradual- 
ly over  a considerable  space,  when  measured  in  the  radiation  wave 
length.  The  oscillatory  motion  of  the  electron  would  tnen  slowly 
subside,  and  its  final  path  would  not  be  very  different  in  directior 

13.  J.J.  Thomsen,  "Electricity  and  Matter",  Ch. 3,  p 53-70,(1906). 

Phil.  Mag.,  Vol.19,  p 335,  Feb. 1910. 


c 45 

from  its  original  path.  It  would  seem,  then,  that  the  scattering  ef- 
fect, due  to.  continuous  wave  fronts  would  probably  be  too  small  to 

be  detected. 

If,  however,  we  postulate  any  kind  of  a wave  theory  in  which 
the  wave  front  is  discontinuous  as  J.J,  Thomson1^  and  A.  Einstein1^' 
have  done,  it  is  evident  at  once  that  an  appreciable  scattering  ef- 
fect would  be  expected  under  the  conditions  of  the  experiment. 

Why  the  velocity  of  the  electron  should  be  decreased  when  it 
passes  through  short  electromagnetic  waves  is  difficult  to  see  in  th: 
light  of  our  present  theories.  So  far  as  is  known  the  usual  form  of 
the  undulating  theory  cannot  give  an  explanation. 

Other  research  work  should  be  done  on  the  two  phenomena  dis- 
covered. Ts?o  experiments  might  be  suggested.  First,  a straight 
beam  might  be  used  and  permitted  to  fall  on  a very  small,  movable 
slit  behind  which  should  be  placed  an  insulated  Faraday  cylinder  at- 
tached tc  an  electrometer.  The  rate  of  collection  of  charge  could 
be  measured  for  any  given  position  of  the  slit  with  the  radiation 
alternately  off  and  on.  When  the  slit  was  near  the  edge  of  the  beam, 
if  scattering  occurred  with  radiation  present,  the  charge  would 
build  up  more  rapidly  than  when  radiation  was  absent.  Second,  the 
other  experiment  could  be  performed  ’with  a device  similar  to  a Braun 
tube.  The  electron  beam  could  be  made  very  narrow  by  passing  it 
through  a hole  in  a diaphragm,  after  which  it  would  pass  through  an 
alternating  magnetic  field.  The  electron  beam  would  then  produce  a 
phosphorescent  line  on  a will  emit  e or  calcium,  tungstate  screen.  The 
cross  hairs  of  an  observing  telescope  could  then  be  set  on  the  end 
of  the  luminous  line.  If  the  length  of  the  line  changed  when 

13.  loc.  cit. 

14.  A.  Einstein,  Phys.  seitsohr,  , V.18,  p 131,  ,1917.  


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46 " 

radiation  was  turned  on,  it  would  be  evident  that  a velocity  change 
bad  occurred  in  the  beam. 

VIII  SUMMARY 

From  the  experimental  work  just  presented  under  the  operating 
conditions  described  above,  the  following  conclusions  may  be  drawn: 

1.  When  a strong  beam  of  radiation  of  wave  lengths  from  8000 
to  1500  Angstrom  units  fell  across  a stream  of  rapidly  moving  elec- 
trons, there  were  indications  of  a slight  decrease  in  the  velocity 
of  the  electron.  This  effect,  however,  was  smaller  than  the  errors 
of  measurement. 

3.  With  the  above  radiation  wave  lengths  the  evidence  is  very 
strong  that  there  was  a scattering  of  the  electrons  in  the  beam. 

5.  When  hard  X-rays  were  used  instead  of  the  radiation  given 
in  1,  there  was  a distinct  decrease  in  the  velocity  of  tile  moving 
electrons,  as  is  shown  by  the  decrease  in  the  diameter  of  the  elec- 
tron trace  (Fig. 6 and  7). 

4.  It  was  also  found  that  X-rays  caused  a decided  scattering 
of  the  electrons  in  the  beam. 

The  author  wishes  to  recognize  the  help  received  from  the  early 
rd  of  Professor  C.T.  Xnipp  on  this  problem,  and  to  express  his 
thanks  to  him  for  his  advice  and  aid  throughout  the  research,  and  to 
Professor  A.P.  Carman  for  the  facilities  of  the  department. 


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VITA 


Claude  J erome  Lapp  was  born  June  24,  1693,  near  Smiths  Creek, 
Michigan.  He  received  his  elementary  education  in  the  public  schoolii 
of  Saint  Clair  County,  Michigan,  and  his  secondary  training  in  the 
Richmond  High  School,  Richmond,  Michigan.  In  September,  1313,  he 
entered  Albion  College,  where  he  received  the  degree  of  Bachelor  of 
Arts  in  June,  1917.  From  September  to  December  1917  he  was  a scholar 
in  physics  at  the  University  of  Illinois,  and  from  December,  1917, 
to  December,  1916,  served  in  the  Aviation  Section  of  the  Bureau  of 
Aircraft  Production  at  the  Bureau  of  Standards.  In  January,  1919, 
he  returned  to  the  University  of  Illinois  and  from  it,  in  1S20,  re- 
ceived the  degree  of  Master  of  Arts.  He  has  held  the  position  of 
Assistant  in  Physics,  University  of  Illinois,  1919,  1919-3$  and  In- 
structor in  Physics  in  the  Summer  School,  1919,  1930. 


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